1. Field of the Invention
The present invention relates to a thermal transfer dye image-receiving sheet. More particularly, the present invention relates to a sheet for recording thereon thermally transferred dye images in a medium color reproduction, at a high resolution, and with a high tone reproduction.
2. Description of the Related Arts
Currently there is enormous interest in the development of new types of color printers capable of recording clear images or pictures, for example, relatively compact thermal printing systems, especially sublimating dye-thermal transfer printers.
In the sublimating dye-thermal transfer printing system, colored images or pictures are formed by superposing thermally transferred yellow, magenta and cyan colored images or pictures in the form of a number of dots, to reproduce colored images or pictures having a continuous hue and color density.
In the sublimating dye thermal transfer printing system, an ink sheet composed of a base film and a sublimating dye layer formed on the base film is superposed on a dye image-receiving sheet composed of a support sheet, and a dye image-receiving layer formed on the support sheet in such a manner that the sublimating dye layer of the ink sheet comes into contact with the dye image-receiving layer of the dye image-receiving sheet, and the ink sheet is locally heated by heat supplied from a thermal head of the printer in accordance with electrical signals corresponding to the images or pictures to be printed, whereby portions of the sublimating ink in the ink sheet are thermally transferred to the dye image-receiving layer to provide colored images in a predetermined pattern and having a predetermined color density (darkness).
Also, in a thermal melting ink transfer printing system, it is possible to print continuous tone full color images on an image-receiving sheet by using a special ink sheet and by thermally transferring a portion of ink in the special ink sheet to the image-receiving sheet through stepwise heating by a thermal head.
It is known that the conventional image-receiving sheet or a substrate sheet for the image-receiving sheet is made from a paper sheet comprising, as a principal component, a cellulose pulp or a surface-smoothed paper sheet, but the conventional paper sheet comprising as a principal component, the cellulose pulp is not satisfactory as a thermal transfer image-receiving sheet capable of recording uniform, continuous tone images thereon, even when the conventional paper sheet is surface-smoothed.
Especially, in a thermal transfer printing system in which the amount of an ink melt to be transferred is controlled by the heat supplied from the thermal head and the sublimating dye thermal transfer printing system, the uniformity in the ink or dyereceiving property of the image-receiving layer in the image-receiving sheet greatly influences the reproducibility of the images. Therefore, when the conventional image-receiving sheet is used, sometimes the resultant solid print has an unevenness in the darkness (color density) thereof, and the transfer of dots is not stable, and thus it is difficult to provide satisfactory continuous tone colored images on the sheet.
To eliminate the above-mentioned disadvantages, an attempt was made to provide, as a substrate sheet, a synthetic paper sheet consisting of a biaxially drawn multilayer polyolefin film comprising, as a principal component, a mixture of a polyolefin resin, for example, a polypropylene resin with an inorganic pigment, and to then form an image-receiving layer on the above-mentioned substrate sheet.
In an image-receiving sheet for a sublimating dye thermal transfer printer, usually a dye image-receiving layer comprising, as a principal component, a polyester resin is formed on the above-mentioned substrate sheet. This type of image-receiving sheet is advantageous in that the sheet has a uniform thickness, a satisfactory flexibility and softness, and a smaller thermal conductivity than that of the conventional paper sheet comprising a cellulose pulp, and thus can receive, images having a high uniformity and color density.
Nevertheless, where the biaxially oriented multilayer film comprising, as a principal component, a polypropylene resin, is used as a substrate sheet, the resultant image-receiving sheet is disadvantageous in that, when images are recorded on the sheet by using a thermal head, the remaining stress in the substrate sheet derived from a drawing process applied to the polypropylene resin sheet is released, and thus the image-receiving sheet is locally shrunk to generate curls and wrinkles in the sheet. These curls and wrinkles hinder the smooth conveyance of the image-receiving sheet through the printing system, and the resultant print has a significantly lower commercial value. Particularly, in the sublimating dye thermal transfer printing system in which a large amount of heat is necessary for the dye-transferring operation, the above-mentioned disadvantages become prominent.
To eliminate the above-mentioned disadvantages, for example, unevenness of received images, by not employing the thermoplastic substrate sheet, Japanese Unexamined Patent Publication No. 62-21590 discloses an attempt to provide a barrier layer comprising an organic polymeric material and formed on a substrate paper sheet.
Nevertheless, this type of image-receiving sheet is disadvantageous in that, to provide printed high quality images, the image-receiving surface must have a very high smoothness, and if the surface smoothness is unsatisfactory, an even transfer of the ink or dye is not obtained, and thus the resultant transferred images have an uneven color density.
U.S. Pat. No. 4,774,224 to Eastman Kodak Co. discloses that the surface smoothness or roughness of the barrier layer comprising the organic polymeric material and formed on the substrate paper sheet has a great influence on the uniformity in color density and gloss of the images formed on the image-receiving layer. Particularly, the direct interdependency between the surface smoothness of the organic polymeric material barrier layer and the uniformity of the transferred images is poor, and when the surface smoothness of the barrier layer is too high, the barrier layer surface exhibits a poor adhesion to the image receiving layer. Further, when the image receiving layer is coated on the barrier layer, sometimes undesirable streaks are formed thereon. Also, it was found that the substrate paper sheet, which naturally has a high rigidity, causes a lowering of the close adhesion between the image-receiving layer and the thermal head, and thus the uniformity of the transferred images on the image-receiving sheet is lowered. To prevent the formation of uneven images, the thermal head must be brought into close contact with the image-receiving layer, under an increased contact pressure, and this close contact of the thermal head under a high pressure shortens the durability (operating life) of the thermal head.
As mentioned above, generally, when a paper sheet comprising, as a principal component, a cellulose pulp is used as a substrate sheet, the resultant image-receiving sheet has a relatively low sensitivity for receiving ink or dye images. To eliminate this disadvantage, an attempt was made, as disclosed in Japanese Unexamined Patent Publication No. 1-97690, to provide a shielding layer comprising a polyethylene resin and formed between the substrate paper sheet and the image-receiving layer. Nevertheless, the resultant image-receiving sheet exhibits a lower sensitivity for receiving transferred ink or dye images than that of the above-mentioned image-receiving sheet in which the substrate sheet consists of a monoaxially or biaxially drawn multilayer film comprising, as a principal component, a polypropylene resin. Therefore, there is a strong demand for the provision of an image-receiving sheet having a high sensitivity.
Furthermore, since the image-receiving sheet is used in the form of a cut sheet, a proper rigidity is an important factor when ensuring a smooth conveyance of the cut image-receiving sheet through the printing system. Also, to evenly produce clear and sharp images transferred to the image-receiving sheet in accordance with the amount of thermal energy, a close contact of the thermal head with the image-receiving layer surface is very important.
Accordingly, where a laminate paper sheet comprising a fine paper sheet and a polyethylene coating layer formed on the fine paper sheet is used as a substrate sheet, if the laminate paper sheet has a low rigidity, the resultant image receiving sheet often causes a jam in the system, or is incorrectly supplied as two or three sheets at the same time, or if the rigidity of the laminate paper sheet is too high, the close contact between the thermal head and the image-receiving layer of the resultant image-receiving sheet is not satisfactory, and thus the uniformity of the transferred images is lowered.
Therefore, there is a strong demand for the provision of a new type of image-receiving sheet able to be smoothly conveyed through the thermal transfer printing system and have uniform colored images formed thereon.